Donald Trump said he will release his tax returns ... on one condition.

"I will release my tax returns — against my lawyer's wishes — when [Hillary Clinton] releases her 33,000 emails that have been deleted," he said at Monday night's debate, prompting cheers from some members of the crowd.

Clinton responded by suggesting Trump might be reluctant to disclose his tax returns because they may reveal that he is not as rich or charitable as he says he is.

Clinton and other Democrats have called on Trump to release his tax returns, which most Republican nominees since 1970 have done. Recent polls have shown most voters think Trump should release his tax returns, as well.

The documents could cast light on how much Trump has really given to charity and resolve questions about whether he received a discounted tax rate in the past. Trump has claimed he cannot release his tax returns because he is under audit, though the IRS has said that an audit would not bar him from releasing the returns.

The Clinton family released its 2015 tax return in August, showing that it paid an effective federal tax rate of 34.2 percent and an effective combined tax rate of 43.2 percent.

Markets seemed to react to Clinton making headway against her GOP rival as the debate progressed.

U.S. stock index futures erased losses to trade higher amid the debate, and the Mexican peso reversed losses to trade more than 1 percent stronger against the U.S. dollar.